1. Technical Field
United States of America generally to the field of well logging. More particularly, this disclosure relates to tools and methods for measuring electrical properties of geological formations. Still more specifically, this disclosure relates to tools and methods for measuring the resistivity of a formation at relatively large radial distances from the borehole.
2. Description of the Related Art
Induction and propagation resistivity tools have been used for many years to measure the resistivity of geological formations surrounding a borehole in the presence of borehole fluids that may have invaded the formations Induction logging tools measure the resistivity (or, more accurately, its inverse, conductivity) of the formation by inducing eddy currents in the formations in response to an AC transmitter signal. The eddy currents induce secondary magnetic fields that in turn induce voltages in receiver antennas. Because the magnitudes of the eddy currents depend on formation conductivities, the magnitudes of the received signals thus reflect the formation conductivities
A typical induction tool includes at least two, and preferably three, induction arrays having different spacings between the transmitters and the receivers to provide different depths of investigation. An early example of a dual array tool is disclosed in U.S. Pat. No. 3,067,383. A minimal configuration of such a tool includes two arrays having different transmitter-receiver spacings to measure two different depth of investigation: a deep array (ILD) and a medium array (ILM). The deep array is designed to “see” beyond the zone invaded by mud filtrate in order to provide true formation resistivity (Rt) In the prior art, the transmitters and receivers are common to one tool.
To determine the minimum parameters (the invaded zone resistivity, Rxo, the resistivity of the uninvaded zone, Rt, and the radius of invasion, r1) of a formation invaded by drilling fluids, at least three measurements at different depths of investigation are required. Therefore, a third shallow measurement (a shallow measurement) has been provided by a focused electrode array placed on the tool. One such tool is disclosed in U.S. Pat. No. 3,329,889. The shallow measurement, together with the medium (ILM) and deep (ILD) measurements provided by the induction arrays, provide sufficient data to solve for formation resistivities in a formation having a simple invasion profile.
However, such a tool may not provide sufficient data for the determination of formation properties such as anisotropy or when the invasion profile is complex. Therefore, multi-array tools having non-axially aligned antennas have been introduced for the determination of formation resistivity in formations with mote complex invasion profiles Examples of multi-array tools include those disclosed in U.S. Pat. No. 5,157,605. The multiple arrays, having fixed spacings between the transmitter and the receiver, can provide measurements at fixed depths of investigation. Therefore, when mud invasion occurs to different extents (radii) in different formation layers, sufficient data may still be provided to solve for the true formation resistivity
A significant drawback to prior art tools is the fixed spacing between transmitters and receivers within a single tool, limiting the measurements to fixed depths of investigation.